1. Field of the Invention
The present invention relates to a light source driving device to drive plural light sources. In addition, the present invention also relates to an image processing device, an image reading device and an image forming apparatus using the light source driving device.
2. Description of the Related Art
There are light sources in which plural chip light emitting diodes (LEDs) are arrayed. When driving such light sources, it is effective to drive as many LEDs as possible using one driver. Therefore, many commercial LED driver ICs (hereinafter referred to as LED drivers) have a voltage boosting function to boost a supply voltage to supply the boosted voltages to such LEDs. Since LEDs are semiconductor parts, they have a good combination of luminous efficiency and response, and can emit high-intensity light with a relatively low power consumption. In addition, LEDs have an advantage such that it is substantially unnecessary to wait for stabilization of the quantity of light emitted thereby, i.e., it is substantially unnecessary to take a wait time before performing scanning using the LEDs.
However, since no power is needed in such conventional LED drivers (and their peripheral circuitries) except for a LED lighting period in which the LEDs are in a lighting state (i.e., the LEDs operate as loads), a voltage boosting operation cannot be performed except for the period. Therefore, when pulse width modulation (PWM) lighting control is performed as illustrated in FIG. 11(b), the voltage boosting operation is divided (i.e., multiple voltage boosting operations have to be performed at intervals), and the voltage boosting operation spends a time, which is 1/D times an operating time (t1) in a case of regular lighting operation illustrated in FIG. 11(a), wherein D represents the duty and is greater than 0 and less than 1. Therefore, it is necessary for such conventional LED drivers to spend a relatively long wait time from lighting of LEDs to start of a scanning operation, thereby causing a problem in that the first copy time is delayed, resulting in deterioration of throughput (productivity) of the image forming operation.
When voltage boosting is performed while always lighting LEDs in order to perform the voltage boosting operation at a high speed, the LEDs emit an excessive quantity of light. In this case, charge coupled device image sensors (hereinafter referred to as CCDs) receive an excessive quantity of light, and it is likely that output characteristics of all of the parts of the circuit such as CCDs reach saturation points thereof, thereby causing a problem in that it takes a considerable amount of time until the parts are recovered or a part damaging problem in that the parts are broken due to generation of an excessive voltage. Thus, the high-speed light-quantity rising property of LEDs cannot be fully exhibited.
In attempting to reduce power consumption and to improve operation stability of a booster circuit, a technique is proposed in which when the battery voltage is higher than a predetermined voltage, a dummy load is connected in series with a load, and when the battery voltage is not higher than the predetermined voltage, the dummy load is shorted by a switch.
However, the technique has a drawback in that since the LED of the circuit is lighted, the problem in that the parts such as CCDs and parts of the signal processing circuit have the saturated states and excessive voltage is generated cannot be solved.
For these reasons, the present inventors recognized that there is a need for a light source driving device which drives plural light sources such as LEDs and which performs a voltage boosting operation at a high speed without forming an excessive voltage for parts of the circuit receiving light emitted by the light sources.